Problem 59 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following statements is correct with respect to the basic character? (a) \(\mathrm{PH}_{3}=\mathrm{NH}_{3}\) (b) \(\mathrm{PH}_{3}>\mathrm{P}\left(\mathrm{CH}_{3}\right)_{3}\) (c) \(\mathrm{P}\left(\mathrm{CH}_{3}\right)_{3}>\mathrm{PH}_{3}\) (d) \(\mathrm{PH}_{3}>\mathrm{NH}_{3}\)

Step-by-Step Solution

Answer

Option (c) \(\mathrm{P}(\mathrm{CH}_{3})_{3}>\mathrm{PH}_{3}\) is correct.

Step 1: Understand the Concept

The basic character of a compound is determined by its ability to donate lone pairs of electrons. In general, the stronger the ability of a molecule to donate its lone pair, the more basic it is.

Step 2: Evaluate the Compounds

Among the given compounds, consider the lone pairs on the central atom. - The lone pair on nitrogen in \(NH_3\) is held by a smaller, more electronegative atom compared to phosphorus in \(PH_3\), making \(NH_3\) more basic. - In \(P(CH_3)_3\) and \(PH_3\), we note that the methyl groups in \(P(CH_3)_3\) donate electron density towards phosphorus, thereby enhancing its ability to donate loner pair electrons, increasing its basic character.

Step 3: Compare Options

Analyze each option:- (a) \(\mathrm{PH}_{3}=\mathrm{NH}_{3}\): Incorrect, \(\mathrm{NH}_{3}\) is more basic than \(\mathrm{PH}_{3}\).- (b) \(\mathrm{PH}_{3}>\mathrm{P}(\mathrm{CH}_{3})_{3}\): Incorrect, \(\mathrm{P} (\mathrm{CH}_{3})_{3}\) is more basic than \(\mathrm{PH}_{3}\) due to the electron-donating effects of the methyl groups.- (c) \(\mathrm{P}(\mathrm{CH}_{3})_{3}>\mathrm{PH}_{3}\): Correct, consistent with our previous analysis.- (d) \(\mathrm{PH}_{3}>\mathrm{NH}_{3}\): Incorrect, \(\mathrm{NH}_{3}\) is more basic.

Key Concepts

Lone Pair Donation

Lone pair donation is a crucial concept in understanding the basicity of a molecule. Basicity refers to a compound's ability to donate a pair of electrons. When a molecule donates its lone pair of electrons to bond with a proton (or another electron-deficient species), it is acting as a base. Generally, the more readily a molecule can donate its lone pair, the stronger its basic character.
For example, in ammonia ( NH_3 ), nitrogen donates a lone pair of electrons to form bonds, giving it a relatively high basic character. This is because nitrogen is small and holds the lone pair tightly, making it more effective at donating it. In contrast, phosphorus in phosphine ( PH_3 ) is larger, resulting in a weaker hold on its lone pair, leading to a lower ability to donate and thus, a lower basicity. Remember that the smaller and more electronegative the atom bearing the lone pair, the stronger the base tends to be.

Electronegativity

Electronegativity is the tendency of an atom to attract electrons towards itself. The more electronegative an atom, the more it can hold onto its electrons. In the context of basicity, it's essential to know how electronegativity affects a molecule's ability to donate lone pairs.
In our examples, nitrogen is more electronegative than phosphorus. This means nitrogen in ammonia ( NH_3 ) can retain its electrons better than phosphorus in phosphine ( PH_3 ). However, because nitrogen holds onto electrons tightly, it also means it can donate them more efficiently to other atoms needing electrons, increasing its basicity.  NH_3  can act as a stronger base compared to  PH_3  due to this reason. Understanding electronegativity helps compare the basicity of different molecules, as seen in how ammonia is more basic than phosphine.

Molecular Structure Effects on Basicity

The structure of a molecule can have significant effects on its basicity. In molecules like  P(CH_3)_3 , we see that the presence of methyl groups influences its basicity through inductive effects. Methyl groups are known to donate electron density through their carbon-hydrogen bonds, augmenting the electron density on the central atom they are attached to, in this case, phosphorus.
This increase in electron density enables phosphorus to donate its lone pair more easily, which enhances the basicity of trimethylphosphine ( P(CH_3)_3 ) over phosphine ( PH_3 ). By comparing different molecular structures, one can predict how such modifications influence the ability of a molecule to act as a base. The impact of additional groups, especially those that donate electrons like alkyl groups, is crucial for predicting and understanding the basic nature of a molecule.